The original World Trade Center Building 7 (WTC 7) was a 47-story
office building located immediately to the north of the main World
Trade Center (WTC) complex. Completed in 1987, it was built on top of
an existing Con Edison substation and located on land owned by The
Port Authority of New York and New Jersey.

When
did WTC 7 collapse?

On Sept. 11, 2001, WTC 7 endured fires for almost seven hours, from
the time of the collapse of the north WTC tower (WTC 1) at 10:28:22
a.m. until 5:20:52 p.m., when WTC 7 collapsed.

What
caused the fires in WTC 7?

Debris from the collapse of WTC 1, which was 370 feet to the south,
ignited fires on at least 10 floors in the building at its south and
west faces. However, only the fires on some of the lower floors—7
through 9 and 11 through 13—burned out of control. These
lower-floor fires—which spread and grew because the water supply to
the automatic sprinkler system for these floors had failed—were
similar to building fires experienced in other tall buildings. The
primary and backup water supply to the sprinkler systems for the
lower floors relied on the city’s water supply, whose lines were
damaged by the collapse of WTC 1 and WTC 2. These uncontrolled
lower-floor fires eventually spread to the northeast part of WTC 7,
where the building’s collapse began.

How
did the fires cause WTC 7 to collapse?

The heat from the uncontrolled fires caused steel floor beams and
girders to thermally expand, leading to a chain of events that caused
a key structural column to fail. The failure of this structural
column then initiated a fire-induced progressive collapse of the
entire building.

According to the report’s probable collapse sequence, heat from the
uncontrolled fires caused thermal expansion of the steel beams on the
lower floors of the east side of WTC 7, damaging the floor framing on
multiple floors.

Eventually, a girder on Floor 13 lost its connection to a critical
column, Column 79, that provided support for the long floor spans on
the east side of the building (see Diagram 1). The displaced girder
and other local fire-induced damage caused Floor 13 to collapse,
beginning a cascade of floor failures down to the 5th floor. Many of
these floors had already been at least partially weakened by the
fires in the vicinity of Column 79. This collapse of floors left
Column 79 insufficiently supported in the east-west direction over
nine stories.

The unsupported Column 79 then buckled and triggered an upward
progression of floor system failures that reached the building’s
east penthouse. What followed in rapid succession was a series of
structural failures. Failure first occurred all the way to the roof
line—involving all three interior columns on the easternmost side
of the building (79, 80, 81). Then, progressing from east to west
across WTC 7, all of the columns failed in the core of the building
(58 through 78). Finally, the entire façade collapsed.

Diagram
1—Typical WTC 7 floor showing locations of columns (numbered). The
buckling of Column 79 was the initiating event that led to the
collapse of WTC 7. The buckling resulted from fire-induced damage to
floors around column 79, failure of the girder between Columns 79 and
44, and cascading floor failures.

What
is progressive collapse?

Progressive collapse is defined as the spread of local damage from a
single initiating event, from structural element to element,
eventually resulting in the collapse of an entire structure or a
disproportionately large part of it. The failure of WTC 7 was an
example of a fire-induced progressive collapse.

Progressive collapse did NOT occur in the WTC towers, for two
reasons. First, the collapse of each tower was not triggered by a
local damage or a single initiating event. Second, the structures
were able to redistribute loads from the impact and fire-damaged
structural components and subsystems to undamaged components and to
keep the building standing until a sudden, global collapse occurred.
Had a hat truss that connected the core columns to the exterior frame
not been installed to support a TV antenna atop each WTC tower after
the structure had been fully designed, it is likely that the core of
the WTC towers would have collapsed sooner, triggering a global
collapse. Such a collapse would have some features similar to that of
a progressive collapse.

How did the collapse of WTC 7 differ from the
collapses of WTC 1 and WTC 2?

WTC 7 was unlike the WTC towers in many respects. WTC 7 was a more
typical tall building in the design of its structural system. It was
not struck by an aircraft. The collapse of WTC 7 was caused by a
single initiating event—the failure of a northeast building column
brought on by fire-induced damage to the adjacent flooring system and
connections—which stands in contrast to the WTC 1 and WTC 2
failures, which were brought on by multiple factors, including
structural damage caused by the aircraft impact, extensive
dislodgement of the sprayed fire-resistive materials or fireproofing
in the impacted region, and a weakening of the steel structures
created by the fires.

The fires in WTC 7 were quite different from the fires in the WTC
towers. Since WTC 7 was not doused with thousands of gallons of jet
fuel, large areas of any floor were not ignited simultaneously as
they were in the WTC towers. Instead, separate fires in WTC 7 broke
out on different floors, most notably on Floors 7 to 9 and 11 to 13.
The WTC 7 fires were similar to building contents fires that have
occurred in several tall buildings where the automatic sprinklers did
not function or were not present.

Why did WTC 7 collapse, while no other known
building in history has collapsed due to fires alone?

The collapse of WTC 7 is the first known instance of a tall building
brought down primarily by uncontrolled fires. The fires in WTC 7 were
similar to those that have occurred in several tall buildings where
the automatic sprinklers did not function or were not present. These
other buildings, including Philadelphia's One Meridian Plaza, a
38-story skyscraper that burned for 18 hours in 1991, did not
collapse due to differences in the design of the structural system.

Factors contributing to WTC 7’s collapse included: the thermal
expansion of building elements such as floor beams and girders, which
occurred at temperatures hundreds of degrees below those typically
considered in current practice for fire-resistance ratings;
significant magnification of thermal expansion effects due to the
long-span floors in the building; connections between structural
elements that were designed to resist the vertical forces of gravity,
not the thermally induced horizontal or lateral loads; and an overall
structural system not designed to prevent fire-induced progressive
collapse.

Some
people have said that a failure at one column should not have
produced a symmetrical fall like this one. What’s your answer to
those assertions?

WTC 7’s collapse, viewed from the exterior (most videos were taken
from the north), did appear to fall almost uniformly as a single
unit. This occurred because the interior failures that took place
did not cause the exterior framing to fail until the final stages of
the building collapse. The interior floor framing and columns
collapsed downward and pulled away from the exterior frame. EVIDENCE
There were clues that internal damage was taking place, prior to the
downward movement of the exterior frame, such as when the east
penthouse fell downward into the building and windows broke out on
the north face at the ends of the building core. The symmetric
appearance of the downward fall of the WTC 7 was primarily due to the
greater stiffness and strength of its exterior frame relative to the
interior framing.

In videos, it appears that WTC 7 is descending in free fall,
something that would not occur in the structural collapse that you
describe. How can you ignore basic laws of physics? WTC 7 did
not enter free fall. According to NIST analysis of WTC 7 video, the
building collapsed 18 stories in 5.3 seconds. If the building
exhibited free fall, this process would have taken just 3.9 seconds.
The actual collapse time exceeded the free fall time by 40 percent.

Does
this mean there are hundreds or thousands of unsafe tall buildings
with long span supports that must be retrofitted in some way? How
would you retrofit a building to prevent this problem?

While the partial or total collapse of a tall building due to fires
is a rare event, NIST strongly urges building owners, operators, and
designers to evaluate buildings to ensure the adequate fire
performance of structural systems. Of particular concern are the
effects of thermal expansion in buildings with one or more of the
following characteristics: long-span floor systems, connections that
cannot accommodate thermal effects, floor framing that induces
asymmetric forces on girders, and composite floor systems, whose
shear studs could fail due to differential thermal expansion (i.e.,
heat-induced expansion of material at different rates). Engineers
should be able to design cost-effective fixes to address any areas of
concern identified by such evaluations.

Several existing, emerging, or even anticipated capabilities could
have helped prevent the collapse of WTC 7. The degree to which these
capabilities improve performance remains to be evaluated. Possible
options for developing cost-effective fixes include:

More robust connections and framing systems to better resist effects
of thermal expansion on the structural system.

Structural systems expressly designed to prevent progressive
collapse. Current model building codes do not require that buildings
be designed to resist progressive collapse.

Better thermal insulation (i.e., reduced conductivity and/or
increased thickness) to limit heating of structural steel and
minimize both thermal expansion and weakening effects. Insulation
has been used to protect steel strength, but it could be used to
maintain a lower temperature in the steel framing to limit thermal
expansion.

Improved compartmentation in tenant areas to limit the spread of
fires.

NIST is recommending that building standards and codes be
strengthened beyond their current intent to achieve life safety to
prevent structural collapse even during infrequent building fires
like those in WTC 7 when sprinklers do not function, do not exist, or
are overwhelmed by fire.

Did
investigators consider the possibility that an explosion caused or
contributed to the collapse of WTC 7?

Yes, this possibility was investigated carefully. NIST concluded that
blast events inside the building did not occur and found no evidence
supporting the existence of a blast event.

In addition, no blast sounds were heard on the audio tracks of video
recordings during the collapse of WTC 7 or reported by witnesses.
According to calculations by the investigation team, the smallest
blast capable of failing the building’s critical column would have
resulted in a sound level of 130 decibels (dB) to 140 dB at a
distance of at least half a mile, if unobstructed by surrounding
buildings. This sound level is consistent with a gunshot blast,
standing next to a jet plane engine, and more than 10 times louder
than being in front of the speakers at a rock concert.

For the building to have been prepared for intentional demolition,
walls and/or column enclosures and fireproofing would have to be
removed and replaced without being detected. Preparing a column
includes steps such as cutting sections with torches, which produces
noxious and odorous fumes. Intentional demolition usually requires
applying explosive charges to most, if not all, interior columns, not
just one or a limited set of columns in a building.

Is
it possible that thermite or thermate contributed to the collapse of
WTC 7?

NIST has looked at the application and use of thermite and has
determined that its use to sever columns in WTC 7 on 9/11/01 was
unlikely.

Thermite is a combination of aluminum powder and a metal oxide that
releases a tremendous amount of heat when ignited. It is typically
used to weld railroad rails together by melting a small quantity of
steel and pouring the melted steel into a form between the two rails.

To apply thermite to a large steel column, approximately 0.13 lb of
thermite would be needed to heat and melt each pound of steel. For a
steel column that weighs approximately 1,000 lbs. per foot, at least
100 lbs. of thermite would need to be placed around the column,
ignited, and remain in contact with the vertical steel surface as the
thermite reaction took place. This is for one column … presumably,
more than one column would have been prepared with thermite, if this
approach were to be used.

It is unlikely that 100 lbs. of thermite, or more, could have been
carried into WTC 7 and placed around columns without being detected,
either prior to Sept. 11 or during that day.

Given the fires that were observed that day, and the demonstrated
structural response to the fires, NIST does not believe that thermite
was used to fail any columns in WTC 7. NON SEQUITER

Analysis of the WTC steel for the elements in thermite/thermate would
not necessarily have been conclusive. The metal compounds also would
have been present in the construction materials making up the WTC
buildings, and sulfur is present in the gypsum wallboard used for
interior partitions. mmm

An emergency responder caught in the building between the 6th and
8th floors says he heard two loud booms. Isn’t that evidence that
there was an explosion? The sound levels reported by all
witnesses do not match the sound level of an explosion that would
have been required to cause the collapse of the building. If the two
loud booms were due to explosions that were responsible for the
collapse of WTC 7, the emergency responder—located somewhere
between the 6th and 8th floors in WTC 7—would not have been able to
survive the near immediate collapse and provide this witness account.

Did
fuel oil systems in WTC 7 contribute to its collapse?

No. The building had three separate emergency power systems, all of
which ran on diesel fuel. The worst-case scenarios associated with
fires being fed by ruptured fuel lines—or from fuel stored in day
tanks on the lower floors—could not have been sustained long
enough, could not have generated sufficient heat to weaken critical
interior columns, and/or would have produced large amounts of visible
smoke from the lower floors, which were not observed.

As background information, the three systems contained two 12,000
gallon fuel tanks, and two 6,000 gallon tanks beneath the building’s
loading docks, and a single 6,000 gallon tank on the 1st floor. In
addition one system used a 275 gallon tank on the 5th floor, a 275
gallon tank on the 8th floor, and a 50 gallon tank on the 9th floor.
Another system used a 275 gallon day tank on the 7th floor.

Several months after the WTC 7 collapse, a contractor recovered an
estimated 23,000 gallons of fuel from these tanks. NIST estimated
that the unaccounted fuel totaled 1,000 ±1,000 gallons of fuel (in
other words, somewhere between 0 and 2,000 gallons, with 1,000
gallons the most likely figure). The fate of the fuel in the day
tanks was unknown, so NIST assumed the worst-case scenario, namely
that they were full on Sept. 11, 2001. The fate of the fuel of two
6,000 gallon tanks was also unknown. Therefore, NIST also assumed the
worst-case scenario for these tanks, namely that all of the fuel
would have been available to feed fires either at ground level or on
the 5th floor.

Why
did the investigation take so long to complete?

The overall NIST investigation began on Aug. 21, 2002. Early in the
investigation, a decision was made to complete studies of the two
tower collapses (WTC 1 and WTC 2) before fully proceeding on the WTC
7 investigation. A major technical conference on the draft reports on
WTC 1 and WTC 2 occurred on Sept. 13-15, 2005. The time between the
technical conference on the WTC towers report and the issuance of
this draft WTC 7 report is approximately three years, comparable to
the length of a typical investigation of an aircraft crash.

The WTC 7 investigation was an extensive, state-of-the-art
reconstruction of the events that affected WTC 7 and eventually led
to its collapse. Numerous facts and data were obtained, then combined
with validated computer modeling that is believed to be close to what
actually occurred. A single computer simulation of the structural
response to fires took about eight months to complete on powerful
computing workstations and clusters.

Did debris from the collapse of WTC 1 cause damage
to WTC 7’s structure in a way that contributed to the building’s
collapse?

The debris caused structural damage to the southwest region of the
building—severing seven exterior columns—but this structural
damage did not initiate the collapse. The fires initiated by the
debris, rather than the structural damage that resulted from the
impacts, initiated the building’s collapse after the fires grew and
spread to the northeast region after several hours. The debris impact
caused no damage to the spray-applied fire resistive material that
was applied to the steel columns, girders, and beams except in the
immediate vicinity of the severed columns. The debris impact damage
did play a secondary role in the last stages of the collapse
sequence, where the exterior façade buckled at the lower floors
where the impact damage was located. A separate analysis showed that
even without the structural damage due to debris impact, WTC 7 would
have collapsed in fires similar to those that occurred on Sept. 11,
2001. None of the large pieces of debris from WTC 2 (the south tower)
hit WTC 7 because of the large distance between the two buildings.

Would WTC 7 have collapsed even if there had been
no structural damage induced by the collapse of the WTC towers?

Yes. Even without the structural damage, WTC 7 would have collapsed
from the fires that the debris initiated. The growth and spread of
the lower-floor fires due to the loss of water supply to the
sprinklers from the city mains was enough to initiate the collapse of
the entire building due to buckling of a critical column in the
northeast region of the building.

Why did WTC 7’s sprinkler systems fail during
the fires?

The sprinkler systems did not fail. The collapse of WTC 1 and WTC 2
damaged the city water main. The water main served as both the
primary and backup source of water for the sprinkler system in the
lower 20 floors. Therefore, the sprinkler system could not function.
In contrast, the sprinklers and standpipes on the building’s middle
levels (21st floor through 39th floor) and upper levels (40th floor
through 47th floor) received water from two large overhead storage
tanks on the 46th floor, and used the city’s water mains as a
backup.

How
hot did WTC 7’s steel columns and floor beams get?

Due to the effectiveness of the spray-applied fire-resistive material
(SFRM) or fireproofing, the highest steel column temperatures in WTC
7 only reached an estimated 300 degrees C (570 degrees F), and only
on the east side of the building did the steel floor beams exceed 600
degrees C (1,100 degrees F). However, fire-induced buckling of floor
beams and damage to connections—that caused buckling of a critical
column initiating collapse—occurred at temperatures below
approximately 400 degrees C where thermal expansion dominates. Above
600 degrees C (1,100 degrees F), there is significant loss of steel
strength and stiffness. In the WTC 7 collapse, the loss of steel
strength or stiffness was not as important as the thermal expansion
of steel structures caused by heat.

Did
the electrical substation next to WTC 7 play a role in the fires or
collapse?

No. There is no evidence that the electric substation contributed to
the fires in WTC 7. The electrical substation continued working until
4:33 p.m. on Sept. 11, 2001. Alarms at the substation were monitored,
and there were no signals except for one event early in the day. No
smoke was observed emanating from the substation.

Special elements of the building’s construction—namely trusses,
girders, and cantilever overhangs, which were used to transfer loads
from the building superstructure to the columns of the electric
substation (over which WTC 7 was constructed) and foundation
below—also did not play a significant role in the collapse.

Why
were there no fatalities from the collapse of WTC 7?

Several factors contributed to the outcome of no loss of life—or
serious injuries—in WTC 7. The building had only half the number of
occupants on a typical day—with approximately 4,000 occupants—at
the times the airplanes struck the towers. Occupants had recently
participated in fire drills. The occupants, alerted by the attacks on
WTC 1, WTC 2, and the Pentagon, began evacuating promptly. Evacuation
of the building took just over an hour, and the process was complete
before the collapse of the first WTC tower (WTC 2). Emergency
responders provided evacuation assistance to occupants. No emergency
responders were harmed in the collapse of WTC 7 because the decision
to abandon all efforts to save WTC 7 was made nearly three hours
before the building fell.

Why didn't the investigators look at actual steel samples from WTC
7? Steel samples were removed from the site before the NIST
investigation began. In the immediate aftermath of Sept. 11, debris
was removed rapidly from the site to aid in recovery efforts and
facilitate emergency responders’ efforts to work around the site.
Once it was removed from the scene, the steel from WTC 7 could not be
clearly identified. Unlike the pieces of steel from WTC 1 and WTC 2,
which were painted red and contained distinguishing markings, WTC 7
steel did not contain such identifying characteristics.

Your entire investigation included no physical evidence. How can
you be so sure you know what happened?

In general, much less evidence existed for WTC 7 than for the two WTC
towers. The steel for WTC 1 and WTC 2 contained distinguishing
characteristics that enabled it to be identified once removed from
the site during recovery efforts. However, the same was not true for
the WTC 7 steel. Certainly, there is a lot less visual and audio
evidence of the WTC 7 collapse compared to the collapses of the WTC 1
and WTC 2 towers, which were much more widely photographed.

Nonetheless, the NIST investigation of WTC 7 is based on a huge
amount of data. These data come from extensive research, interviews,
and studies of the building, including audio and video recordings of
the collapse. Rigorous, state-of-the-art computer methods were
designed to study and model the building’s collapse. These
validated computer models produced a collapse sequence that was
confirmed by observations of what actually occurred. In addition to
using its in-house expertise, NIST relied upon private sector
technical experts; accumulated copious documents, photographs and
videos of this disaster; conducted first-person interviews of
building occupants and emergency responders; analyzed the evacuation
and emergency response operations in and around WTC 7; performed
computer simulations of the behavior of WTC 7 on Sept. 11, 2001, and
combined the knowledge gained into a probable collapse sequence.

Did
WTC 7 conform to building and fire codes?

The team found that the design of WTC 7 in the 1980s was generally
consistent with the New York City building code in effect at that
time.

WTC 7's designers intended its stairwells to evacuate nearly 14,000
occupants, anticipated at the time to be the maximum occupancy of the
building. Though the stairwell’s capacity was overestimated, it was
adequate for evacuating the building’s actual maximum occupancy of
8,000, and more than adequate to evacuate the approximately 4,000
occupants who were in the building on Sept. 11.

What improvements to building safety have been recommended as a
result of the WTC 7 investigation?

NIST has made one new recommendation and reiterated 12
recommendations from the investigation of the WTC towers.

The new recommendation involves explicitly evaluating buildings to
ensure the adequate fire safety performance of the structural system.
Of particular concern are the effects of thermal expansion in
buildings with one or more of the following characteristics:

long-span floor systems

connections not designed for thermal effects

floor framing that induces asymmetric forces on girders, and

composite floor systems whose shear studs could fail due to
differential thermal expansion (i.e., heat-induced expansion of
material at different rates in different directions).

Typical floor span lengths in tall office buildings are in the range
of 40 ft. to 50 ft. This range is considered to represent long span
floor systems. Thermal effects (e.g., thermal expansion) that may be
significant in long-span buildings may also be present in buildings
with shorter span lengths depending on the design of the structural
system.

This investigation is the first to show how fire can cause
progressive collapse in a building. It is also the first to show that
under certain conditions thermal expansion effects—rather than loss
of strength and stiffness due to fire—can lead to structural
collapse. It is the first to analyze a building’s response behavior
and determine its collapse sequence by integrating detailed
models/simulations of debris impact damage, fire growth and spread,
thermal analysis, collapse initiation, and collapse propagation—up
to global collapse. This was an analysis of unprecedented
complexity—an end-to-end computer run for the WTC towers on some
powerful computers took about two months while a similar run for WTC
7 took about eight months, or about four times as long. NIST expects
that the tools developed from this investigation, as well as the
knowledge obtained from it, will aid in the development of more
robust building design practice and in studies of future building
collapse processes. These expanded tools and derived, validated, and
simplified analysis approaches can guide practitioners and prevent
future disasters.

Why
is NIST studying the collapse of WTC 7?

The NIST investigation of WTC 7 was conducted under the National
Construction Safety Team (NCST) Act, as part of its overall building
and fire safety investigation of the World Trade Center disaster. The
act gives NIST the responsibility for conducting fact-finding
investigations of building failures that resulted in substantial loss
of life or that posed significant potential of substantial loss of
life. NIST has no regulatory authority under the NCST Act.

How can I provide comments on the report?

NIST welcomes comments on the draft report and
recommendations—available online at http://wtc.nist.gov. Comments
must be received by noon Eastern Daylight Time on Sept.15, 2008.
Comments may be submitted via:

Have
the recommendations from NIST’s investigation of the WTC towers led
to any changes in building codes, standards, and practices?

The first comprehensive set of eight model building code changes
based on recommendations from NIST’s investigation of the WTC
towers were adopted by the International Building Code in 2007.

A second set of eight model building code changes based on NIST’s
recommendations from its investigation of the WTC towers were
approved by technical committees and are awaiting approval, along
with potential appeals on several other code changes, at the Final
Action Hearing for the 2009 edition of the International Building
Code.

NIST’s recommendations from its investigation of the WTC towers
also have spurred actions to develop new provisions/guidelines within
other standards, codes, and industry organizations, such as: the
National Fire Protection Association, the American Society of
Mechanical Engineers, ASTM International, the American Society of
Civil Engineers, and the Council on Tall Buildings and Urban Habitat.

What specific code changes based on recommendations from NIST’s
investigation of the WTC towers have been approved for inclusion in
the International Building Code?

The eight specific code changes adopted in the International Building
Code based on recommendations from NIST’s investigation of the WTC
towers include:

1. An additional exit stairway for buildings more than 420 feet in
height.

2. A minimum of one fire service access elevator for buildings more
than 120 feet in height.

3. Increased bond strength for fireproofing (nearly three times
greater than currently required for buildings 75-420 feet in height
and seven times greater for buildings more than 420 feet in height).

4. Field installation requirements for fireproofing to ensure that:

installation complies with the manufacturer’s instructions;

the substrates (surfaces being fireproofed) are clean and free of
any condition that prevents adhesion;

testing is conducted to demonstrate that required adhesion is
maintained for primed, painted or encapsulated steel surfaces; and

the finished condition of the installed fireproofing, upon complete
drying or curing, does not exhibit cracks, voids, spalls,
delamination or any exposure of the substrate.

5. Special field inspections of fireproofing to ensure that its
as-installed thickness, density and bond strength meet specified
requirements, and that a bonding agent is applied when the bond
strength is less than required due to the effect of a primed, painted
or encapsulated steel surface. The inspections are to be performed
after the rough installation of mechanical, electrical, plumbing,
sprinkler and ceiling systems.

6. Increasing by one hour the fire-resistance rating of structural
components and assemblies in buildings 420 feet and higher. (This
change was approved in a prior edition of the code.)

7. Explicit adoption of the “structural frame” approach to fire
resistance ratings that requires all members of the primary
structural frame to have the higher fire resistance rating commonly
required for columns. The primary structural frame includes the
columns, other structural members including the girders, beams,
trusses, and spandrels having direct connections to the columns, and
bracing members designed to carry gravity loads.

8. Luminous markings delineating the exit path (including vertical
exit enclosures and passageways) in buildings more than 75 feet in
height to facilitate rapid egress and full building evacuation.